Glaxo SmithKline has begun a named patient programme in the UK providing GW433908, the improved formulation of amprenavir (for further information on amprenavir, click here). This will make the drug available prior to licensing for people who need a protease inhibitor with a low pill burden.
GW433908, also known as fosamprenavir, is a pro-drug of amprenavir, which means that it is converted into amprenavir during absorption. The compound is better absorbed, meaning that similar levels of amprenavir can be achieved by taking fewer capsules. GW433908 is supplied in 700mg capsules, and is dosed with 100mg ritonavir to boost levels of amprenavir.
An experimental dose of two capsules once daily, together with 100mg of ritonavir, is currently being tested.
The drug will be made available for patients who, in the opinion of their doctor, require a protease inhibitor with a low pill count, including patients currently taking amprenavir. Doctors should contact Glaxo Smith Kline; patient enquiries cannot be accepted. GW433908 will cost £319.73 for 35 days supply (70 capsules).
There are no data to show that treatment with GW433908 results in a superior viral load response, but the compound does have an equivalent effect to amprenavir. Pharmacokinetic studies have shown that GW433908 treatment is associated with higher trough levels and lower peak levels (potentially reducing the risk of side effects associated with high peak levels of the drug, or increasing resistance associated with sub-optimal drug levels). Study APV2001 showed a trend towards a lower rate of adverse events in those who received GW433908.
Further data to support the use of GW433908 are currently being gathered in three studies comparing GW433908 alone or with ritonavir to nelfinavir and to lopinavir/ritonavir.